In electrography an image comprising an electrostatic field pattern, usually of non-uniform strength, (also referred to as an electrostatic latent image) is formed on an insulative surface of an electrographic element by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on a surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of an electrostatic field pattern on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
Where high resolution toner images are desired, development is typically carried out electrophoretically by contacting the latent image with a liquid electrographic developer comprising particulate toner material of very small size (e.g., less than 1 micrometer) dispersed in an electrically insulating organic carrier liquid, such as an isoparaffinic hydrocarbon liquid. The toner particles migrate to and deposit on areas of the insulative surface having relatively high or low field strength, depending upon the triboelectric characteristics of the toner material and the polarity and strength of an external electrical field usually applied across the electrographic developer and element during the development process. An imagewise deposit of toner particles is thus formed on the insulative surface and can be fixed in place on the surface by application of heat or other known methods (depending upon the nature of the toner particles to be fixed) or can be transferred in some cases to another surface to which it then can be similarly fixed.
Many types of toner materials are known to be useful in liquid electrographic developers. Such toners comprise at least a binder component (often in combination with a colorant material), in particulate form. A number of properties can be identified, which would be highly desirable to have in optimum toner binder particles. Optimum toner binder particles would be of very small size (each particle having an average diameter less than one micrometer) to enable formation of colloidal or near-colloidal dispersions and to enable high resolution imaging and capability of forming void-free smooth-surfaced deposits having good transparency, toughness, and resistance to abrasion or other degradation by solvents or other dry or oily materials. Optimum toner binder particles would be insoluble in but swellable by and, thus, dispersible in, electrically insulating organic carrier liquids of choice (e.g., isoparaffinic hydrocarbon liquids) and would exhibit good dispersion stability therein, either by themselves or with the aid of dispersing agents and stabilizers that can be dispersed or dissolved in the liquids. Optimum toner binder particles would comprise thermoplastic polymeric materials having relatively high molecular weight (e.g., at least 10.sup.6 g/mole) and a relatively low glass transition temperature (Tg) when in contact with and swollen by the carrier liquid, thus exhibiting an amorphous, yet still particulate, character that would enable them to bind themselves and other desirable developer addenda into seemingly continuous film deposits on insulative surfaces, either at ambient temperatures or with minimal application of heat; yet the particles would have a relatively higher Tg in the absence of the carrier liquid, so that when the carrier liquid is evaporated from deposits of the particles on a surface, and the deposits are allowed to cool, they will not flow from their original areas of deposition at room temperature; they will be fixed in place and be tough and abrasion-resistant. Optimum polymeric toner binder particles would also contain a significant degree of crosslinking in the polymeric material in order to further enhance structural integrity and solvent-resistance of the resultant deposits.
It will be readily appreciated, however, that fashioning such optimum toner binder particles having a desirable degree of the above-noted properties would be a very difficult task, especially since means for imparting some of these properties to the particles often work at cross-purposes with means for imparting other of these properties.
For example, the desire to make polymeric particles satisfactorily dispersible in certain carrier liquids by making them insoluble in but swellable by the carrier liquids, has led some workers in the prior art to fashion certain copolymers derived from a mixture of monomers. Some of the monomers in such a mixture may be chosen for their ability to form homopolymers that would be soluble or at least swellable in the carrier liquid of choice, in order to impart swellability to the copolymer intended to be produced. Other of the monomers in the mixture may be chosen for their ability to form homopolymers that would be insoluble in the carrier liquid, in order to enable the intended copolymeric particles to exhibit the necessary property of insolubility in the carrier liquid. This is the approach disclosed, for example, in U.S. Pat. Nos. 3,788,995; 3,849,165; and 4,171,275. The balancing of amounts of solubilizing and insolubilizing monomers, however, can affect more than just the dispersibility of the particles. For example, while it may be fairly simple to create particles that are insoluble, swellable, and dispersible by using a relatively high ratio of solubilizing to insolubilizing monomers, the relatively large amount of solubilizing monomers may produce a copolymer that is disadvantageously susceptible to attack by solvents that it may come into contact with during use; or the copolymer may be particularly susceptible to oily abrasion (e.g., degradation by rubbing contact with human skin and oils). The U.S. patent disclosures noted above do not address these potential problems. Those disclosures also do not suggest that the copolymers should have any crosslinking to improve structural integrity and solvent resistance. Crosslinking could inherently upset the property balance needed to achieve good dispersibility (crosslinking generally leading to decreased dispersibility). Furthermore, those disclosures teach that the copolymers should be produced in bulk by techniques such as solution polymerization, whereupon inefficient and time-consuming milling procedures are then required to achieve the desired submicrometer particle sizes. Optimum toner binder particles would be capable of being synthesized initially in the desired particle sizes, thus avoiding the need for such milling procedures.
Another approach to obtaining polymeric toner binder particles for liquid electrographic developers is disclosed in U.S. Pat. No. 4,306,009. It involves use of so-called "gelatex" particles comprising interpenetrating networks of two different polymers that are physically entangled but not chemically bonded to each other. Requirements of insolubility, swellability, and dispersibility in carrier liquids are met by fashioning one of the polymers (called a latex) to be insoluble in the carrier liquid and fashioning the other polymer (called a gel) to be soluble or "on the borderline of solubility" in the carrier.
All specific examples of the gel polymers in that patent are fashioned from mixtures of monomers that contain a relatively high percentage of solubilizing monomers. One or both of the polymers can be a copolymer and can contain trace amounts of crosslinking in order to achieve permanence of the physical entanglement. It is said that the gel polymer portion of the particles can be produced from a mix of monomers of which up to 1.2 percent by weight can comprise crosslinking vinyl monomers. Higher amounts of crosslinking apparently would detract from the gel polymer's solubilizing function and ability to be penetrated by the monomers of the insoluble latex polymer during its synthesis within the gel polymer network. The preparation of those gelatex materials involves a fairly lengthy procedure of synthesizing the gel polymer and then the latex polymer entangled within it and apparently produces a bulk material, which must then be milled to desired size. Because there is some degree of crosslinking in the gel polymer portion of the gelatex, it would be very difficult to mill the material so that the sizes of all particles would be less than one micrometer; the patent states that the final gelatex particles have a mix of sizes distributed from about 0.1 to about 1.5 micrometers. Thus, these gelatex particles are not the optimum toner binder particles noted previously herein as desirable, because they have a larger than optimum distribution of sizes, their degree of crosslinking is limited, they must be fashioned from more than one polymer, and they must be milled to final size.
It is, therefore, evident that there is a need to provide polymeric particles that could serve as optimum toner binder particles having all of the desirable characteristics noted above. That need is satisfied by the invention described in copending U.S. patent application Ser. No. 858,464, filed Apr. 30, 1986, now U.S. Pat. No. 4,708,923 (the disclosure of which is hereby incorporated herein by reference and will be hereinafter referred to, for convenience, as the Alexandrovich et al disclosure).
While the more generic invention of Alexandrovich et al does indeed provide polymeric particles having all of the optimum toner binder characteristics noted above, many of those particles require combination with dispersing aids in order to be capable of being dispersed in some organic liquids. It would be even more desirable to provide polymeric particles also having the property of being capable of being readily dispersed by themselves in many organic liquids of choice (e.g., in Isopar G.TM. or heptane) and of remaining well dispersed therein for lengthy periods (e.g., at least 6 months), without the aid of dispersing and dispersion-stabilizing agents. Such particles would be desirable, because one would then be able to use them for all the purposes described in the Alexandrovich et al disclosure, without concern for whether dispersing aids, that might otherwise also need to be used, would have any adverse effects on other aspects of embodiments in which the particles are intended to be employed (e.g., adverse effects on aspects such as transparency or electrical properties). Furthermore, it has been found that even though all the particles of Alexandrovich et al can be successfully dispersed in many organic liquids with the aid of dispersing agents, in many specific cases those dispersions do not remain stable for as long as six months, even with the aid of dispersion stabilizers; i.e., in some cases they have been found to provide a shorter-than-desirable shelf life when dispersed.
The present invention avoids these shortcomings by providing crosslinked vinyl polymer particles having all of the desirable characteristics noted for the generically described particles of Alexandrovich et al and, additionally, having the further desirable characteristics of being readily dispersible by themselves in a paraffinic liquid such as heptane to form a dispersion that will be stable for at least six months under typical room conditions.